1. Field of the Invention
The present invention relates to a horizontal deflection circuit for use in a television display apparatus that incorporates a CRT (Cathode-ray Tube).
2. Description of the Related Art
Generally, in a video display incorporating a CRT, the raster position, which the electron beams emitted from the electron guns can reach, is displaced with respect to the screen. This is because the CRT has productive variation in raster position. As a consequence, the image displayed by the video display distorts in both vertical direction and horizontal direction, or parts of the raster fail to appear in the screen.
Most video displays, each incorporating a CRT, comprises a centering circuit that corrects the raster position. To move the raster position, a direct current is supplied to the deflection yokes. Usually the raster position can be easily corrected in the vertical direction, in view of the structure of the vertical deflection circuit. In the horizontal deflection circuit, which is usually a resonator circuit, however, it is often difficult to supply a direct current to the deflection yoke. In the television receivers, for example, the centering circuits 100 and 110 shown in FIGS. 1 and 2 have been used hitherto. More precisely, the centering circuits shown in FIGS. 1 and 2 are used in the horizontal deflection circuit of the so-called xe2x80x9cdiode modulator type,xe2x80x9d which is illustrated in FIG. 3.
The centering circuit 110 shown in FIG. 2 can be used in ordinary horizontal deflection circuits. However, it is large in circuitry size, and its manufacturing cost is inevitably high.
The centering circuit 100 shown in FIG. 1 can be manufactured at lower cost than any other types. However, it is not easy to incorporate the centering circuit 100 into a horizontal deflection circuit, for the following reason. As shown in FIG. 1, the centering circuit 100 comprises a choke coil 102, a capacitor 103, an S-shaped capacitor 104, a switch 106, and two diodes 107 and 108. The choke coil 102 connects the capacitor 103 to a power supply 101. The diodes 107 and 108 are selectively connected by the switch 106 to the node of the S-shaped capacitor 104 and a horizontal deflection yoke 105.
The switch 106 may be operated, selecting the diode 108 connected to the S-shaped capacitor 104. In this case, the diode 108 is turned on when the voltage across the S-shaped capacitor 104 falls below the voltage across the capacitor 103. Then, the voltage across the capacitor 103 falls along the voltage across the S-shaped capacitor 104. Hence, while the diode 108 remains on, a direct current can be supplied to the S-shaped capacitor 104 without imposing large influence on the resonance that the horizontal deflection circuit is achieving. The centering circuit 100 supplies a current equivalent to the current supplied to the S-shaped capacitor 104, from the power supply 101 to the horizontal deflection yoke 105, thereby moving the raster in the horizontal direction.
The switch 106 may selects the diode 107 that conducts a current in the direction opposite to the direction the diode 108 conducts a current. In this case, the diode 107 is turned on when the voltage across the S-shaped capacitor 104 rises above the potential of the capacitor 103. Then, a current is supplied to the capacitor 103 from the S-shaped capacitor 104, without affecting the resonance that the horizontal deflection circuit is achieving. As a result, the centering circuit 100 moves the raster in the opposite direction.
In the centering circuit 100 it is required that the average voltage of the power supply 101 be equal to the average voltage across the S-shaped capacitor 104. It would otherwise be impossible to achieve stable horizontal movement of the raster, either to the left or to the right.
In the horizontal deflection circuit described above, the voltage across the S-shaped capacitor 104 does not much depend on the voltage of the power supply 101 in most cases. The voltage across the S-shaped capacitor 104 is often higher than the voltage of the power supply 101. When a current flows throughout the centering circuit 100 if the diode 107 is selected to supply a current from the S-shaped capacitor 104 to the capacitor 103, a large current is therefore supplied to the horizontal deflection yoke 105. That is, the average voltage of the power supply 101 and the average voltage across the S-shaped capacitor 104 cannot be equal at all in the centering circuit 100. If a large current is supplied to the horizontal deflection yoke 105, the raster will be moved more than is necessary in the horizontal direction. In some cases, the raster position cannot be adjusted to an appropriate position.
To solve this problem, the voltage of the power supply 101 connected by the choke coil 102 to the capacitor 103 may be applied from another power supply of a higher voltage, not from the power supply that drives the deflection yoke of the horizontal deflection circuit. The use of another high voltage power supply will, however, render the centering circuit 100 too expensive to be used in ordinary television receivers.
The present invention has been made in view of the foregoing. The object of the invention is to provide an inexpensive horizontal deflection circuit that can effect a stable horizontal movement of raster.
To attain the object, a horizontal deflection circuit according to the invention has a centering circuit designed to change a direction of supplying a current to a deflection yoke, thereby to move a raster in a horizontal direction and, hence, adjust the position of the raster. The centering circuit comprises: a diode, of which cathode is connected to a node of an S-shaped capacitor and the deflection yoke; a capacitor connected at one end to the anode of the diode and at the other end to the ground; a switching element connected in parallel to the diode; and switching element control means for opening and closing the switching element, thereby to control a current flowing from the S-shaped capacitor to the capacitor and to control, ultimately, a current to be supplied to the deflection yoke.
The horizontal deflection circuit can therefore continuously adjust the raster position in the horizontal direction.